[VadeRetro]

Maybe there's some naturalistic theory as to how a moon gets a wall around it on a great circle arc which can be seen from space, but I've never heard such a theory.

Let me take your virginity on this one. Impact. Liquification. Cooling and hardening. Small object, low gravity, tectonically inactive, high-relief structures can stay essentially forever. (Or until obliterated by subsequent impacts.)

There are lots of maria-filled craters on the moon. I'm surprised you've never seen it.

[tomzz]

You might want to take a harder look at the picture. The little moon has a wall all the way around it which neatly divides the moon into two equal halves. If that were an easy thing to have happen, all by itself, then why don't other bodies in our system have such walls? Again, there is no rational theory as to how that could happen naturally.